Modern drilling operations used to create boreholes in the earth for the production of oil, gas and geothermal energy typically employ rotary drilling techniques. In rotary drilling, a borehole is created by rotating a tubular drill string having a drill bit secured to its lower end. As drilling Proceeds, additional tubular segments are added to the drill string to deepen the hole. While drilling, a pressurized fluid is continually injected into the drill string. This fluid passes into the borehole through one or more nozzles in the drill bit and returns to the surface through the annular channel between the drill string and the walls of the borehole. The drilling fluid carries the rock cuttings out of the borehole and also serves to cool and lubricate the drill bit.
The most common type of bit used in rotary drilling is known as a rotary-cone bit. Rotary-cone bits have two or more spindles at their lower end with each spindle serving as an axle for a rotary cutting element known as a cone. The spindles and cones are configured so that the cones bear on the bottom of the borehole. As the drill string and bit are rotated, the cones turn on the spindles. The outer face of each cone is provided with steel teeth or tungsten-carbide inserts which penetrate into the bottom of the borehole as the drill string turns, thus deepening the borehole.
A second type of bit, known as a drag bit, does not employ any moving components. Drag bits have a main body into the outer surface of which are embedded extremely hard cutting elements. These cutting elements are typically made of synthetic diamonds. As the drag bit is rotated, the cutting elements scrape against the bottom and sides of the borehole to cut away rock.
All types of drill bits undergo wear in the course of drilling operations. One type of wear is the dulling of the cutting elements. This generally causes the cutting ability and penetration rate of the bit to decrease with increasing usage. This decrease in the penetration rate is readily observable at the surface, permitting the driller to pull the drill string at the appropriate point to replace the bit.
There are other types of wear, not readily apparent at the surface, which have posed longstanding problems for the drilling industry. One of these is loss of gauge. Each drill bit is designed to drill a borehole of a specific gauge (diameter). As drilling progresses, the gauge maintaining Portion of the bit abrades against the borehole wall, decreasing the diameter of the bit. This causes the diameter of the drilled hole to progressively decrease. An undergauge borehole can damage a new drill bit and increase the likelihood of differential pressure sticking of the drill string within the borehole, among other problems. Where a hole is drilled undergauge, it is generally necessary to enlarge the diameter of the hole with a special reaming tool. This is a time-consuming and expensive operation.
A second type of wear is specific to roller-cone bits. In roller-cone drilling operations, the bearing surfaces between each cone and spindle will wear. As these surfaces wear, the cone will begin to rotate eccentrically about the spindle. If drilling continues, the cone may eventually seize or fall off the spindle. If a bit bearing should fail and leave a cone in the wellbore, it is often necessary to withdraw the drill string and suspend drilling operations until the lost cone can be fished from the well. The resulting delay can be very expensive, particularly in offshore wells.
It has long been desired to develop an inexpensive and reliable means for indicating when a bit is about to go undergauge or lose a cone. At present, drillers often elect to replace the bit well before they think it likely that a problem has developed to avoid the possibility of needing to fish a cone or ream the well. These bits are often discovered to have considerable life remaining when they are brought to the surface. If there were some means for indicating when a wear-related problem is about to arise, each bit could be used for its maximum effective life, reducing the time and cost of drilling a well.